Production of silicon dioxide



July 10, 1962 w. HUGHES ETAL PRODUCTION OF SILICON DIOXIDE OriginalFiled March 10. 1958 8 Sheets-Sheet 1 3 I I 5* 22 (a 5 5 July 10, 1962w. HUGHES ETAL PRODUCTION OF SILICON DIOXIDE Original Filed March 10,1958 8 Sheets-Sheet 2 sac/194th? ib I Y v F M walla. 37- Z9. v

Jul 10, 1962 w. HUGHES ETAL 3,043,660

PRODUCTION OF SILICON DIOXIDE Original Filed March 10, 1958 8Sheets-Sheet s 6 5 a m w w 4/ a) 2 4 .\u\ 2 w w E wflwm 2 WW July 10,1962 w. HUGHES ETAL 3,043,660

PRODUCTION OF \SILICON DIOXIDE Original Filed March 10, 1958 v sSheets-Sheet! July 10, 1962 W. HUGHES ETAL PRODUCTION OF SILICON DIOXIDEOriginal Filed March 10. 1958 8 Sheets-Sheet 5 July 10, 1962 w. HUGHESETAL 3,043,660

PRODUCTION OF SILICON DIOXIDE Original Filed March 10. 1958 8Sheets-Sheet 6 July 10, 1962 w. HUGHES ETAL 3,043,660

I PRODUCTION OF SILICON DIOXIDE Original Filed March 10, 1958 8Sheets-Sheet 7 m m F W. HUGHES ETAL PRODUCTION OF SILICON DIOXIDE July10, 1962 Original Filed March 10, 1958 8 Sheets-Sheet 8 United StatesPatent Ofi ice 3,043,660 Patented July 10, 1 962 3,043,660 PRODUCTKGN FSILICON DIQXIDE William Hughes, Fairfield, Stockton-on-Tees, and ArthurWaliace Evans, Nunthorpe, Middlesbrough, England, assignors to BritishTitan Products Company Limited, Biliingham, England, a corporation ofthe United Kingdorn Original application Mar. 10, 1958, Ser. No.720,470. Divided and this application June 2, 1958, Ser. No. 739,418

Claims priority, application Great Britain July 25, 1955 3 Claims. (Cl.23-182) This application is a continuation-in-part of applicationsSerial No. 598,913, filed July 19, 1956, now abandoned, and Serial No.721,579, filed March 14, 1958, and a division of application Serial No.720,470, filed March 10, 1958.

This invention relates to the production of silicon dioxide by thevapour phase oxidation of silicon tetrachloride.

Suitably prepared finely divided silica is becoming increasinglyimportant as a filler and reinforcing agent for natural and syntheticrubbers and for synthetic and natural plastic materials, and also as athickener and suspending agent for various liquid mixtures andsuspensions, and as an agent for other uses.

Processes for the production of finely divided oxides, including silicondioxide, have been suggested in which the corresponding vapourisedhalide, particularly the chloride, is converted to the oxide by variouscombustion processes involving oxidation or hydrolysis at elevatedtemperatures.

These processes, though varying considerably indetail, all require theuse of burners or jet assemblies for feeding the reactant gases andvapours to the reaction space. The apparatus is often furthercomplicated by the need to maintain the reaction temperature and, insome cases, to provide the moisture for the hydrolysis reaction by thesimultaneous combustion of hydrogen, hydrocarbons or other vapourisedfuels. In these processes it is rarely possible to increase theproductionof the apparatus by an increase'in the size of the jets orburners as this usually leads to a deterioration in the quality of. theproduct. Consequently, for large scale production it is necessary to usea large number of similar jets or burners.

It is an object of the present invention to provide a process for themanufacture of silicon dioxide which is highly efiicient and in whichthe reaction temperature can be readily controlled, and which is moreadaptable for large scale production.

In performing the process of the invention by which this object isachieved vapourised silicon tetrachloride is oxidised in a fluidised bedof finely divided solid inert material.

More specifically the process of the invention for the production ofsilicon dioxide comprises establishing a fluidised bed of solid inertparticles, maintaining the temperature of said bed sufficientlyhigh tocause silicon tetrachloride to react with oxygen, while introducingsilicon tetrachloride and oxygen into said bed whereby silicon dioxideis formed and carrying silicon dioxide thus produced away with the gasesleaving the fluidised bed.

The particulate inert solid material constituting the bed in which thereaction is to take place may be selected from sand-like materials, i.e.silica, zircon, mineral rutile, alumina or massive mineral rockmaterials which are resistant to chlorine or chlorine-containingsubstances likely to be present in the course of the oxidation reactiondescribedand at the temperatures encountered. The sand-like material ispreferably substantially entirely composed of particles not less than 76microns in diameter and normally not greater than about one-eighth of anstantially greater than 1000a diameter. It will be appreciated that theterm massive mineral relates to minerals which are of such compactnature that the density of each particle thereof approximates thedensity of a substantially perfect specimen of the material. Thematerial comprising the fluidised bed should be such that it wouldfluidise in an air stream at a temperature of 1000" C. for

100 hours at a velocity five times the minimum fluidising velocity, andthe amount of dust and fine material carried aw-ayin suspension in theemerging air stream would not exceed 5 percent (preferably one percentor below) of the material originally present in the bed.

The molar ratio of oxygen to silicon tetrachloride is preferably withinthe range 1:1 to 2:1. Higher oxygen ratios, e.g. up to 5:1, may be usedbut complete reaction of the silicon tetrachloride is normally achievedwithin the preferred range. Molar ratios less than 1:1 obviously giveincomplete oxidation of the silicon tetrachloride.

The gases may be used in a relatively dry condition, or, for control inthe reaction, certain proportions of moisture may be tolerated,particularly in the oxygen stream. It will be appreciated that thepresence of large proportions of moisture is desirably to be avoided,since the presence of moisture may convert the chlorine to hydrochloricacid. The latter is generally detrimental to the process, in thathydrochloric acid cannot so readily be re-used for the purpose ofchlorination, as normally conducted in accordance with the preferredprocess of this invention, which conveniently may utilise thechlorination of ferrosilicon for the production of further supplies ofsilicon tetrachloride. The latter reaction, whether hydrogen chloride orchlorine is used, is highly exothermic and it is essential to employ inconjunction therewith a means of indirect cooling which, in effect,normally means the use of metal and hence poses the problem ofcorrosion. Furthermore, in the chlorination of ferrosilicon withhydrogen chloride, hydrogen is formed and this entails the necessity ofseparation from the silicon tetrachloride vapour and involves certainadditional safety precautions. Where the source of silicon tetrachlorideis native silica, it is still more desirable that chlorine be used forchlorination, rather than hydrochloric acid.

The silicon tetrachloride is vapourised by any suitable means prior 'tobeing fed to the bed. The rate of feed of the silicon tetrachloridevapour and oxygen is primarily a function of the size of the apparatus,but there is additionally both an upper and lower limit for successfuloperation. The upper limit arises from the requirement of a sufficientretention time within the fluidised system, this retention time for aconstant rate of feed per unit area being determined by the depth of thebed. Thus if the reaction is not complete in the bed some buildup willoccur on the walls of the reaction chamber above the bed. The lowerlimit arises from the necessity of fluidising the bed.

In the process one at least of the reactants, preferably the air oroxygen, is fed through the base of the reaction vessel containing acolumnar bed of material as defined above so that the gas velocitywithin the reactor is suflicient to maintain the bed in the fluidisedstate. The other reactant may also be fed separately through the base ofthe reactor or maybe fed otherwise such as by injection in gaseous forminto the bed at a point a short distance above the base of the reactorand preferably so as to deliver this reactant in a generally downwarddirection to encounter the rising air or oxygen.

The silicon tetrachloride and oxygen react Within the bed to formsilicon dioxide and chlorine, according to the equation:

Thus it will be seen that the formation of silicon dioxide is not causedby hydrolytic action, as has been the case hitherto in vapour-phaseoxidations, and thus results in the formation of chlorine, rather thanhydrochloric acid, which latter, as has already been mentioned, isgenerally detrimental to the process.

The silicon dioxide is initially produced in the form of finely-dividedparticles entrained in the other product gases, but may be separatedfrom the entraining gases by simple devices as, for example, cyclones,since the particles readily agglomerate to form much larger aerogellikeflocs. The gases may be cooled before passage through the cyclones byvarious procedures including re-circulation of tail gases, and theintroduction of liquid coolants such as chlorine. Other methods includemechanical methods of an indirect nature generally well-known in theart. The silica produced according to the process of this invention is avery voluminous material, as is evidenced by its weight of 2-20 lbs. percu. ft.

When oxygen is used in stoichiometric proportions in the reaction theproduct gas consists almost entirely of chlorine. In this case, afterseparation of the suspended silicon dioxide, the chlorine may be useddirectly for the production of fresh silicon tetrachloride or for otherpurposes. However,,it may be found more convenient to recover thechlorine from the product gases by well known methods such as byrefrigeration or adsorptionin liquids. If air is used as the oxidisinggas the resulting chlorine will be considerably diluted with nitrogenand the mixed gas can then be treated for recovery of chlorine by anywell known means prior to discharge to atmosphere.

The silicon dioxide product as collected consists of fairly coarse flocsof silica gel. This product, shaken with water, may, depending on theefficiency of the chlorine separation, give a pH value of between 1 and2 owing to the presence of acid and/or chlorine contamination from thereaction. The pH value'of such a suspension can be brought up to between4-7 by various means such as washing with water, but the preferredmethod which is more particularly described hereinafter, and whichpreserves the initial character of the material, is to fluidise theproductwith hot air containing ammonia and preferably also water vapouror super-heated steam, both at a temperature above 250 C., preferably at300 C., or

. above.

The temperature of the fluidised bed should be within the range 500 C.to'1300" C. Though the reaction of silicon tetrachloride with oxygen isexothermic, the heat of reaction may not be sufiicient to maintain therequired reaction temperature when the process is worked on a 'smallscale. In this case the required reaction temperature can be attainedand maintained in various ways such as by separately preheating thereaction gases or by external heating of the reactor or by heating thebed of particulate material by indirect heat exchange with a heatingcoil within the bed or by other internal heating means,;or by theadmission of hot inert gases to the bed, or by admitting a'combustiblegas tothe bed which burns with excess oxygen to give the required heatof reaction. The reaction gasesmay be premixed before they are fed tothe reactor, but .if. in this case preheating is employed thetemperature of the preheating should not exceed about 500 C. This is nota preferred method of operation as the ducts through which'the gases arefed to the bed will, where in contact with the fluidised bed, normallyattain the temperature of the bed, and there is therefore a likelihoodof premature reaction within and consequent blocking of the feed ducts.

In the accompanying drawings which somewhat schematically illustrateapparatus embodying the invention and capable of being used inpractising processes according tothe invention:

FIGURE 1 shows diagrammatically in sectional elevation the generalnature and layout of a small-scale apparatus equipped with externalheating means;

FIGURE 2 is an enlarged scale detail view in sectional elevation of thepart of the apparatus shown in FIGURE 1 into which reactants areintroduced;

FIGURE 3 is a vertical sectional elevation of a larger apparatusincluding a shaft furnace, a solids feed device and a solids-collectingand cooling device, suitable for autothermal operation;

. FIGURE 4 is a plan view of a detail of FIGURE 3;

FIGURE 5 is an enlargement in vertical elevation of a detail of FIGURE 3slightly modified;

FIGURE 6 is a top plan view from above of FIG- URE 5;

FIGURE 7 shows an enlargement in vertical sectional elevation of amodified detail of FIGURE 3;

FEGURE 8 is a top plan view of FIGURE 7;

FIGURE 9 is a diagrammatic sectional elevation of apparatus forseparating products formed in the apparatus of FIGURE 3;

FIGURE 10 is a diagrammatic sectional elevation of a treating chamberfor solid products obtained from the apparatus of FIGURE 9; and

FIGURE 11 is a flow diagram illustrating the complete process operation.

Performance of the process by using a small-scale apparatus will firstbe described by reference to FIGURES 1 and 2. In this construction, thereactor consists of a vertically disposed silica tube 511 having, say aninternal diameter of two inches and an overall length of 36 inches. Thetube 511 is mounted within an electrical furnace indicated at 512 forapplying heat over the lower two thirds of the length of the tube,

The reactants, i.e. oxygen or air on the one hand and vapourised silicontetrachloride on the other hand, are introduced in the bottom part ofthe tube 511 by means which will be hereinafter described withparticular reference to FIGURE 2.

The reactor tube 511 is charged with silica sand of average particlesize of such that the static depth of the sand bed 513 in the tube is 7or 8 inches.

The top of the tube 511 is connected to a junction piece 520 closed atthe top and having a branch limb 521 which is connected to a shaft 522leading downwards to a collecting vessel 523. The shaft 522 is providedwith a side limb 524 for the withdrawal of product gases, a plug 525 ofglass wool being situated in the side limb 524 to prevent solid materpassing out.

Referring now to FIGURE 2, the bottom of the reactor tube 511 is fittedwith a porous ceramic disc 514 through the centre of which passes asteatite tube 515 extending a short distance above the base of thereactor tube 511 and provided at the top with a cap 516 having at thebottom of a depending skirt portion thereof a porous ceramic disc 517.One or more holes 530 are formed in the top portion of the tube 515 toprovide communication between the interior of the tube 515 and the space53-1 in the cap 516 over theporous disc 517.

Underneath the reactor tube 511 is mounted a block 518 having a hollowspace 532 underlying the porous disc 514. A conduit 519 is fitted intothe bottom of the block to communicate with the space 532. Passingthrough the block 518 and the porous disc 514 is a thermocouple 533 tomeasure the temperature of the silica sand bed.

In operation the reactor tube 511 is heated by the electric furnace 512so that the temperature of the bed of silica sand 513, as measured bythe thermocouple 533, Within the bed is 980 C. Vapourised silicontetrachloride at the rate of 10 ml. of liquid silicon tetrachloride perminute is fed into the tube 515 whence it passes through the hole orholes 530 into the space 531 and then through the porous disc 517 in ageneral downward direction into the bed 513. Air or oxygen is fed intothe bed through the tube 519, space 532 and porous disc 514 in an amountsuch that the molar ratio of oxygen to silicon tetrachloride is 2:1. Theair or oxygen passing into the bed causes turbulences in the bed andbrings it into a fluidised state. This will expand the bed to a heightof about 11 to 12 inches.

' The air or oxygen and the vaporised silicon tetrachloride reacttogether within the bed of fluidised silica sand to produce fineparticle size silica which is transported from the reactor tube in theform of an aerogel by the product gases passing up through the side limb521 of the junction piece 520 and thence into the shaft 522.Flocculation of the particles occurs on leaving the bed and the materialwhich is separated from the product gases in the shaft 522 and fallsinto the collecting vessel 523 consists of fairly coarse flocs of silicagel.

The gel particles were further treated with steam at a temperature of300 C. and the resultant product was compared with the nearestcommercially available fine particle size silica as follows:

Nearest S102 Prod- Commernet of cially Invention Available Silica pHvalue 4.1 4. 4 Surface Area (B.E.'I. method), sq. m./g- 290 191Incorporated in silicone rubber and cured for 15 wins. at 265 F. and1hr. at 300 F.:

Tensile strength, lbs/sq. in 670 530 Hardness 26 26 Incorporated insilicone rubber and cured for 15 mins. at 265 F. and 1 hr. at 300 F.,after ageing for 4 days at 480 F;

Tensile Strength, lbs/sq. in 410 390 Hardness 27 37 When compounded intoa natural rubber the product containing the sample pigment was slightlyslower. curing than that containing the commercially available pigment,probably because of its higher surface area. The ultimate properties,however, were substantially the same.

Good results can be obtained with considerably lower proportions ofoxygen, e.g. a molar ratio of oxygen to silicon tetrachloride of 1.25:1.

The apparatus more particularly described hereinbefore is very usefulwhen it is desired to carry out oxidation of silicon tetrachloride on alaboratory scale. However, when it is desired to carry out the operationon a large scale, the use of external heating should be avoided since,owing to the corrosive nature of the silicon tetrachloride and of thereaction products, the furnace is likely to be constructed of ceramicnon-conductive material, and so external heating is not only uneconomicbut is also ditficult to control in the sense that the temperatureconditions over a large reactor tend to be irregular, and this bringsabout variations in the product. An important advantage of the presentinvention is that it is possible to carry out autothermal oxidation ofsilicon tetrachloride on a large scale in such manner as to avoid thenecessity of external heating, and the consequent variations in theproduct.

As has been indicated above, it is important to minimise variations inthe product, and it is in consequence desirable to distribute thereactant gases uniformly over the cross-section of the reactor furnace.This problem is not of such great importance in small-scale reactors ofthe type which has been specifically described hereinbefore. Withlarge-scale reactors it is far more difficult to obtain uniformconditions of fluidisation with a single port of entry, or withrelatively crude methods of gasdistribution as exemplified hereinbeforeby a porous plate. According to the preferred method of operationaccording to the invention, it is possible to carry out the: reaction ona large scale by introducing the two reactants in regulated amounts, andpreferably separately, through a plurality of ports distributed over thecrosswhich is hereinafter more particularly described, both reactantscontribute to the fluidisation of the-bed, and

thus quickly intermingle and react while within the confines of the bed.Preferably the ports of entry of the two reactants should be alternatedso far as possible.

Accordingly, there is hereinafter described the preferred method ofoperation, namely a process for producing silicon dioxide by reactingsilicon tetrachloride vapour with oxygen in the course of their upwardpassage through a fluidised bed of inert solid material so that thesilicon dioxide which is produced is at least for the most partdischarged from above the bed entrained in outgoing gases, characterizedby the following features:

(a) That the reactants are heated in the bed to the extent required tocause them to react so that external preheating is not required:

(b) That the bed, adequately insulated, contains a suflicient quantityof the inert solid material to conserve from the heat of the exothermicreaction what is necessary to effect continuously said heating of thereactants which are, or at least one of which is, being introduced sorapidly as to fluidise the bed in the desired manner;

(0) That the reactants are introduced into the bed through a pluralityof inlet ducts distributed and mutually arranged with respect to thehorizontal cross-sectional area of the bed so as to enable uniformfluidisation of the tive inlet ducts distributed and arrangedasaforesaid and so as to ensure the intermingling of the respectivereactants required for their inter-reaction to take place within thebed;

(of) That the inlet ducts for the reactants areprovide withconstrictions of predetermined dimensions to ensure that a supply underpressure of the reactants, in their required proportions, isappropriately distributed among the inlet ducts appertaining thereto;and

(e) That each constriction in an inlet duct produces a pressure dropfrom the pressure of the supply ofreactant thereto which is at least onehalf of the pressure drop from the bottom to the top of the fluidisedbed.

As regards (a) above it will be understood that external preheating ofthe reactants is not completely precluded because, in the first place,the silicon tetrachloride will be preheated at least to the extent ofvapourising it and, in the second place, there is no disadvantage, ifconvenient so to do, to use oxygen which is preheated to a moderatelyraised temperature. In fact it is desirable to preheat the oxygen atleast to the extent necessary to prevent condensation of the silicontetrachloride vapour,

eg to a temperature of 50 to C.

As regards (b) above it is obvious that the size of the cross-sectionalarea of the bed is a more important factor than height of the bedbecause increase of height to accommodate the required amount of bedmaterial would unduly increase heat losses apart from requiring largerfluidising forces. Therefore, to achieve the desired autothermaloperation of the process there is a minimum size for the cross-sectionalarea of the bed and we estimate that this means, assuming a cylindricalreaction chamber, that the diameter of the bed must be at least fifteeninches. It may of course be larger but it should be borne in mind, thatin designing for substantially larger diameters, the conserved heat mayexceed What is required to maintain the reaction and that provision forcooling of the reaction zone should therefore be made.

The fluidised bed employed may be as dmcribed hereinbefore as to bedmaterials, particle size, and like details, except that, as has alreadybeen specified, there should be sufficient inert solid material toconserve from the heat of the exothermic reaction at least what isnecessary to maintain continuance of the reaction.

As has already been mentioned, the gaseous reactants aoaaeco arecontinuously introduced into the inert hotbed through a plurality ofinlet ducts to maintain uniformity of reaction throughout the bed. Thevelocity of the gas maintaining the bed in the fluidised state isdesirably between two and fifty times the minimum required forfluidisation, and preferably between three and ten times such minimum.For this purpose, the inlet ducts are provided with the above-mentionedconstrictions, the size of which is so chosen that withthe necessaryrate of gas-flow the pressure-drop across the constrictions is at leastone half, and desirably less than fifty times, the pressure-drop of thegas in passing through the bed, thus affording a substantially even flowof the gaseous reactants over the whole of the bed material.

The pressure drop across the constrictions will generally exceed 2 lbs.per square inch, and thetotal pressure drop across the constrictions andthe bed will generally be above 3 lbs. per square inch but rarely over100 lbs. per square inch.

The temperature of the reactor, when of internal diameter considerablygreater than fifteen inches (say eighteen inches or greater) maybecontrolled, in the sense of being kept down as necessary, by the use ofgaseous coolants as exemplified by chlorine, nitrogen, carbon dioxide orcooled recycled tail gases which may be introduced directly into thefluidised bed, or by liquid chlorine injected into or sprayed upon thebed. In addition, or alternatively, the temperature of the reactor maybe controlled by introducing, progressively, relatively cool sand orother inert bed material into the bed, and correspondingly discharginghot sand from the bed.

The temperature of the fluidised bed, although it may range between 500C. and 1300 C., is preferably maintained within the range of 900 C. to1100 C., the range of 1000 C. to 1050 C. giving especially good results.Under the temperature conditions just specified, other general controlfactors may be varied to maintain the conditions desired. Thus theoxygen gas and silicon tetrachloride vapour will usually be fed to thereactor at a velocity (assuming the reactor to be empty) of from aboutone-quarter to about two feet per second, or higher. Where bed materialprogressively fed into and out of the reactor, the rate of feed mayvary, as illustrated in the examples. But any conditions used must bebalanced for autothermal operation. In general, it may be noted that inany given installation the insulation is fixed, and the oxygen'andsilicon tetrachloride feed is determined at least in part by the amountsrequired to maintain fluidisation. .Under these circumstances thetemperature will usually be kept down within the desired range by feedof extraneous coolant or of bed material as mentioned above.

In a preferred embodiment, the reactor is essentially a vertical shaft,usually cylindrical, and lined internally with chlorine-resistingbrickwork, which, in turn, is protected by an outer shell of insulatingbrick, the Whole being contained within a steel shell, the latterterminated at the top and the bottom with openings corresponding to theshaft-on which arevconstructed extension pieces which are flanged totake a header in the case of the top and a hearth unit to be attached tothe bottom. The latter unit desirably consists of a steel plate,surmounted by a heat-insulating block sealed thereto and itselfsurmounting gas-inlet and gas-supply means. The steel plate contains anumber of apertures spaced uniformly according to a predetermined planin order to provide for the admission of the reactants, and theinsulating block contains a number of bores, in which refractory tubesmay be fitted, to provide passages registering with the apertures. Theapertures in the plate are fitted with gas-inlet means havingconstructions of predetermined size. The passages through the insulatingblock may optionally be provided at their upper ends with devicesdesigned to prevent solids from falling down therethrough but to permitthe flow of gas upwards. Said block functions essentially to insulatefrom the heat of the reactor the metal plate and the gas inlet means andgas-supply devices positioned below. The whole hearth unit assembly isconstructed so as to fit into the base of the furnace shaft. so'that themetal plate supporting the structure may beattached to the lower flangedend of the steel shell of the furnace.

One set of theinlet means is designed for. the admi sion of silicontetrachloride and another set, appropriately neighboured with the firstmentioned set, for the admission of the oxygen. The inlet means forsilicon tetrachlorideinto the appropriate passages may be connected toone or more manifolds or to a windbox, and the inlet means feeding theoxygen may similarly be connected to a separate manifold, or manifolds,or windbox. In either case, it will beclear that the gas-inlet means,preferably Welded on to or into the metal plate,

will be of such length and so fabricated that they may be convenientlyconnected to link with the respective manifolds orwindboxes. With awindbox construction, there may be a plug containing the above-mentionedconstriction at the point of entry to each inlet means. In the casewhere a manifold is used, each inlet means may comprise a pipe with aflanged end connected with a corresponding flanged end of a pipe leadingfrom the manifold, the constriction being present as an orifice in adisc held between the two flanged ends.

A preferred feature is that there should be admission of the oxygenreactant round the Walls of the reactor,

' so far as possible, in order to avoid undue reaction at the staticsurface provided by the wall, as opposed to the dynamic surface providedby the fluidised particles.

Although it is desirable to incorporate as large as possible a number ofgas ports into the base of the reactor, there should not be so manyports as will weaken the base of the reactor. It is also of coursedesirable to make the hearth unit at the base of the reactor asinsulating as possible was to retain the heat of reaction within thefurnace.

An essential feature of this preferred embodiment of the invention isthe use of constrictions of predetermined dimensions in the inlet ductsfor the reactants. These constrictions are an important controllingfactor in the system ofgas distribution, and the dimensions aredetermined having regard to the fluidisation required, the properties,i.e., the density and viscosity,'of the reactant gas, and the amount ofgas which it is desired to admit, taking into account the number ofinlet ducts available. It will be appreciated that the constrictions forthe different reactants may be of ditferent dimensions.

The header plate which is secured to the flanged end at the top of thesteel shell of the furnace may be constructed with two openings, one forthe temporary insertion of a poker or other suitable device to effectinitial heating of the furnace and also for admission of the materialforming the bed, and the otherfor conveying the products of reactionfrom the furnace to suitable cooling, collecting and/or separatingdevices to be described bereinafter.

With the hearth unit affixed, any one of the abovementioned particulateinert solid materials, or a mixture of such materials, is fed into thefurnace to a static depth'desirably of approximately 1-3 feet. It maybemore but this is usually unnecessary. The bed thus formed is thenfluidised by a stream of air fed through the inlets at the base of thereactor, and a pre-ignited gas poker may be inserted into the bed. Inthis way, the furnace may be raised to a temperature of sayapproximately 1000 C., whereupon the gas poker is removed, and the inletthrough which it was inserted suitably sealed. At'this stage theair-stream is shut off and oxygen, or a gas rich in oxygen, is passedinto the furnace through the appropriate inlets. The silicontetrachloride ductings, inlets and passages are, to start with, sweptwith a stream of nitrogen, and then silicon tetrachloride is passedtherethrough, whereupon reaction takes place substantially entirelywithin the bed. The silicon dioxide thus produced is carried up out ofthe bed entrained with the chlorine-containing product gases, and isdesirably led from the furnace through the ducting in the header tosuitable cooling, collecting and/or separating devices described laterherein, which may be of Various types.

The silica, as separated from the gases, eg. by means ofcyclones, isfound still to contain appreciable quantities of adsorbed or combinedchlorine and, depending upon the precise details adopted in the process,possibly some hydrochloric acid in addition. These contaminants, or atleast the undesirable effects thereof, may be removed from the silicondioxide by various means of heat treatment, especially at temperaturesbetween BOO-600 C. This purifying step may be conducted by passing airor other innocuous gas through the material which is either heated insitu or preheated beforehand, e. g. by passage, preferablycounter-current to an airstream, down a horizontal or inclined rotarytube of standard design, or by utilising a fluidised bed technique inwhich cold gas, as for example air, is fed for the purpose offluidisation. An alternative procedure is to employ for this purposepreheated gases as fluidising agents. A prefer-red gas for thisfiuidisation is oxygen, including oxygen-containing gases, which,whether heated beforehand or becoming heated as a result of thefluidisation conditions imposed, may be thereafter conveyed to theoxidation chamber for use in the oxidation of further silicontetrachloride.

' Further, in conducting such after-treatment of the product, the gasesused for removing or counteracting the efiect of the undesirableconstituents by means of a fluidised bed technique maypreferably containsome added basic material, ammonia by choice, with or without watervapour, so as to accelerate the removal or neutralisation of thechlorine in the silicon dioxide. This addition may be accomplishedquantitatively, eg by passing the gases either at room temperature or atan elevated temperature, through a tower in which a controlled amount ofammonia is admitted as a gas, or sprayed as an aqueous solution.

While as indicated, substantially all the silicon dioxide produced iscarried forward entrained within the product gases, a small proportionof the silicon dioxide may adhere to the substrate material comprisingthe bed. Where the accumulation, after a period of time, becomesexcessive, it may be necessary to discharge the bed completely andreplace it, unless, as hereinbefore mentioned, the bed is progressivelyrenovated.

It has already been demonstrated that the heat evolved by the oxidationreaction is utilised to maintain the temperature and is adequate to doso. Thus the chamber should be well insulated and the rate of heat lostto the surroundings should not be greater than the rate at which theheat is evolved. It follows, therefore, that for the process to beautothermal, the reaction chamber will require to be adequatelyfabricated for this purpose, both in regard to size and materials ofconstruction. As has already been stated, it has been found in practicewhen using well-known materials of construction, that a minimum internaldiameter of a cylindrical shaft furnace is about inches. In employing afurnace of 15 inches in diameter it is possible to maintain thetemperature by minor controls such as by slight variations in the rateof feed of the reactants. When, however, furnaces of larger constructionare employed, it is desirable, rather than to employ constructionalmaterial giving less insulation, to introduce into the bed coolingagents, as already indicated, whereby the temperature of reaction iskept down as required.

In a preferred embodiment, fully described hereinafter, cooling iseflected and the temperature of reaction controlled by continuouslyfeeding cool solid inert fluidisable material to the bed to replace acorresponding amount of hot material which is continuously discharged.The

10 amount of discharge and replacement will depend on the temperature ofthe replacement material at the time of feeding and the amount of heatto be removed. Thus to get the maximum heat removal with a minimumamount of discharge and replacement, cold replacement material can beused. In the event, however, of it being desirable at the same time toincrease the purge in the bed, the replacement material maybe fed in atan elevated temperature so as to obtain the same cooling eifect with alarger feed and in consequence a greater purge. It will be appreciatedthat there may be two requirements (a) to cool the bed, and (b) to purgethe bed, and by varying the temperature of the replacement materialthere is a freedom of action in respect of the quantity thereof to beadmitted. By such means, the bed may be progressively renovated, thusovercoming the possible drawback associated with accretion of syntheticsilica on the bed particles.

In FIGURE 3 there is shown by the general reference numeral 1 a furnacechamber lined with chlorine-resisting brickwork 2 supported and lined onthe outside with insulating brickwork 3, the whole being contained in asteel shell 15 which has openings at the top 6 and bottom 7. Onto theseopenings are welded short collars 8, terminating in flanges 9, the wholebeing mounted by means not shown, so that furnace 1 stands vertically.

A metal base plate 10 has surmounting it a ceramic block 11 constructedso that when the base plate 10 is inserted into the bottom opening 7 ofthe furnace 1, it will neatly fit whereby the block 11 serves toinsulate from the shaft of the furnace 1 the base plate '10 below. Thebase plate contains apertures 13 registering with bores 12 in the block11, the apertures 13 and bores 12 being distributed over the plate 10and block 11 in a design which is shown in plan view in FIGURE 4.

In this particular and somewhat simplified design, the bores 12 aresubdivided into (1) a set ofpassages 112 for admission of the silicontetrachloride, the passages 112 being arranged in the form of anoctagon, i.e. there being eight passages surrounding the centre of theblock 11, and (2) a set of passages 212 and 312 for admission of oxygen,these latter passages being arranged in the form of an outer octagon ofpassages 212 and an additional passage 312 in the centre of the block11, the apertures 13 registering with the passages 112, 212 and 312, ashas already been indicated.

The upper parts of the bores in the ceramic block 11 may be fitted withgas-emergent means designed positively to bar ingress of the bedmaterial, and yet to permit the passage of the reactant gases, e.g. 'ofthe type described in British patent specification No. 724,193 andapplication Nos. 4,973/55 and 29,584/56 but it is preferred to operatewithout the use of such devices, and have passages 12 of limiteddiameter such that the reactants may be fed with suificient velocity toprevent solid body material from falling back into the passages. ThusFIGURE 3 shows passages 12 without any such devices.

FIGURE 3 shows an arrangement in which the passages 12 are fed withreactants from a manifold system. A similar system is also shown in moredetail in FIG- URE 5, although in the latter figure, solids non-returndevices in the form of porous caps are shown in the upper portions .15of the passages 12.

One manifold 25 distributes oxygen to passages 212 and 312, wholeanother manifold 26 distributes silicon tetrachloride vapour to passages112. All the passages 112, 212 and 312 communicate with pipes 41 whichare welded to the plate 10 and are fitted with flanges 104 (see FIGURE5) at their lowest extremities. 104 is secured a flange on a pipe 42leading to the manifolds 25 and 26, respectively, for oxygen and silicontetrachloride, a constriction being provided by a machined orifice 47present in a disc 43 being held between 7 the flanges 104 and 105.

To each flange I l FIGURES and 6 also show the provision of gaspermeablesolids-impermeable devices 102, 202 and 302, in the upper portions ofthe passages 112, 212 and 312, the latter being flared so as toaccommodate the devices which prevent solids from falling into thepassages and the gas-feeding systems, while allowing the gas to escapetherethrough. It will be seen that the devices 262 and 302m the oxygeninlet passages 212 and 312, respectively, are of larger size thanthedevices 102 in the silicon tetrachloride passages 112. Instead ofthese devices, other types may be used e.g.. others described in Britishspecification No. 4,973/55, but it is preferred to rely merely on theforce of the fluidising gases to prevent solid material from fallinginto the feed system.

A further modification is shown in FIGURES 7 and 8 Where refractorytubes 400 made for example of an alumino silicate are fitted in thebores in the insulating block 11, and have outlets to the furnace intheir tops as shown at 410. Pipes 41 welded to the plate 10 pass throughthe apertures therein and extend into the tubes 400. Sockets 401 aresecured on the lower ends of the pipes 41 and these receive screw plugs402 having orifice constrictions 403. It will be noted that certain ofthe pipes are coupled to downward extension pipes and that these havethe sockets and plugs at their ends. The plugs of the pipes which arenot extended downwards are open to a windbox 4 Whilst those of theextended pipes are open to a Windbox 405.

Windbox 404 is adapted to receive an oxygen supply through inlet 406,and windbox 405 to receive a silicon tetrachloride supply through inlet407. It will be seen from the plan view of FIGURE 8 that the tubularpassageways to the, furnace for the oxygen are in groups 403 whilstthose for the silicon tetrachloride are in intermediate groups 409.Although. a windbox supply with orificed plugs isshown in FIGURE 7, itwill be appreciated that manifolds, and constrictions formed in orificeddiscs, may be used instead; In fact the pattern of distribution of therespective inlet means shown in FIGURE 8 lends itself conveniently to asupply from manifolds because the latter can be, straight, correspondingto the straight disposition of the passageways for the oxygen andsilicon tetrachloride as seen in FIGURE 8. In that case the manifoldsfor the oxygen and silicon tetrachloride may be supplied in oppositedirections from manifolds, as indicated by the arrows.

Reverting to FIGURE 3, the top 6 of the furnace is covered by a closure40, which is atfixed to the upper flange 9 and which surmounts ablackl40 of insulating ceramic material. This closure is formed toprovide a port 24 for feeding in the solid bed material which sub-'sequently constitutes the bed in operation. The solid bed material isfed from a solids-free device 71 which is shown diagrammatically inFIGURE 3. The solids-feed device consists of a 5 ft. length of steeltube, 6" in internal diameter, with a tapered bottom to which is sealedflange pipe 72, 2. in diameter, communicating with a source ofcompressed air. Above the taper at 73 is afiixedv a perforated plate,carrying holes ,4 in diameter and spaced at half-inch intervals to forma square pattern. The upper portion of the tubing is bisected over alength of 3 feet and the top of the lower portion thereof is sealed witha horizontal steel cover 74. An inclined flanged pipe 70, 2" indiameter, leads directly to the furnace 1 from the lower part of thefeed device at a point just below the cover, A flat steel strip 75 issealed onto the bisected length of tubing, said strip projectingdownwards to about 6" from the base of the tube, measured from 73; thepurpose of this projection being to prevent or minimise the effects ofany backflow of gases from the reactor.

There is also provided a port 126 in the side wall of the furnace 1through which the products of reaction are conveyed to ancillaryapparatus for separation. The ancillary apparatus in the form which isshown in FIGURE 9 consists of a comically-shaped receiving vessel 35into which the products discharged from the port 126 of the furnace areled through a pipe 27 having a centrally-positioned discharge conduit36. .In this vessel, the greater part of the coarse silica agglomeratessettle and may be discharged, periodically or continuously according torequirement, through a valve 28, being aided Where neces sary, byvibratory motion imparted to the sides of receiving vessel 35 by knownmeans. The gases leaving this separator via conduit 29 are conveyed to acyclone or, if necessary, a series of cyclones as represented by cyclone30 wherein any of the finer agglomerates of silica produced may beseparated from the gas stream, which is led 01f through ducting 34. Thefiner material descends through a pipe 32, is collected in a collector33 below the cyclone, and is discharged through valve 31, eitherperiodically or continuously according to requirement. The gases afterbeing stripped of their solid content and usually containing chlorine asthe main constituent, may be re-used directly for chlorination ofsilicon-containing material, as, for example, ferro-silicon, or they maybe passed to conventional equipment for the removal of the chlorineconstituent either by cooling, compression and liquefaction of thechlorine constituent or by absorption of the cooled gases in sulphurchloride or other suitable absorbent from which they may be regeneratedby conventional means.

Solid material discharged from the base of separator 35 via valve 28 orfrom cyclone 30 via valve 31, is collected for subsequent removal of theabsorbed chlorinecontaining gases, either to intermediate storage ordirectly to a particular vessel about to be described, in which thisoperation may be conducted.

One method of accomplishing this object is by means I of a vessel whichin a simple form is shown in FIGURE 10 with the general referencenumeral 51. It comprises a cylindrical container with a perforated base55 through which the gases used for purging are admitted in such a wayas to fluidise a bed of the solid material above it. The container maybe heated externally by a suitable jacket 52 either electrically or byother means, such as a circulating gas or liquid. Gases entering via 53,either. cold or heated, pass into windbox 54 and thence via perforatedplate 55 into bed 56 and, While the gases are flowing, the bed 56 ismaintained desirably at temperatures within the range 300-600 C.

t will be apparent thatthere are various ways in conducting thisoperation. Thus, it may be conducted batchwise or continuously. In thecase of batch treatment, the process is comparatively simple in that thematerial is fed into vessel 51 through a conduit 58 and maintainedtherein while heated for a sufficient period such that the product isessentially purged of its acidity. The gases emerging from top 57 may,after suitable purging of the acidity, be discharged to atmosphere. Theproduct after this purging treatment is discharged through outlet 59 byopening valve 60.

In a continuous process, the solids are continually fed through conduit58, and solids are discharged through conduit 61 controlled by valve 62.

FIGURE 10 shows the chamber divided by means of a partition 63 so thatmaterial fed continuously through the conduit 58 and fluidised in thechamber cannot immediately discharge through 61 but by passage throughthe bed section 64 it is purged by the fluidising gases and by passageto the lower level of the partition 63 into the section of the bed 65 itwill ultimately pass upward to be discharged through the conduit 61. Asexplained earlier, the gas used for drying and purging may be preheatedor may be the sole source of heating for efiecting the treatment of thesilica material which constitutes the bed. It may further consist of airbut is preferably oxygen containing entrained ammonia, with or withoutwater vapour, in which case the gas is fed up through pipe 53 throughwindbox 54, perforated plate 55 and subsequently emerges from bed 56 viaoutlet 57, having purged the product, and is then available, if desired,for admission to the manifold 25 (FIGURE 3) and thence through thepassages 12 into the reaction chamber 1. The material overflowing fromport '61 is substantially free from combined or adsorbed chlorine andmay have a pH above 3.5 and preferably 4.0-5.0.

Reverting to FIGURE 3, approximately 2 feet from its base, the furnace 1is provided in the interior of the furnace with a conduit 77, which isfabricated in refractory chlorine-resistant brick, and inclined at anangle of about 45 to the vertical. The conduit 77 may either be sealed,or, if it is desired to introduce solid bed material and withdrawsurplus material during operation of the apparatus, the lower (andouter) end of this conduit is connected by means of flanged joint 78 toa side am 170 of a vertical pipe 79, 3" in internal diameter,

sealed into a flanged lid 80 of a mild steel vessel 81, of diameter 8"and height 2 ft., the pipe 79 projecting downwards within the vessel 81to a point approximately 3" above the top of its tapered base. Justbeneath the lower extremity of pipe 79, a stainless steel disc 82, inchthick, is affixed to the sides of vessel 81, said disc being perforatedwith holes of diameter A arranged in a square pattern of side length 2".At a point approximately 6" from the sealed top, vessel 81 is providedwith a pipe 83, which serves as a means of overflow. At the top ofvessel 81 is a small outlet port 84 through which the fluidising gasescan be voided to atmosphere. Through the lower extremity of its taperedbase, vessel 81 is fitted with flanged pipe 85, connected with a sourceof compressed air. The part of the vessel 81 above the perforated disc82 is encased in a steel jacketof conventional design 86, through whicha stream of cold water can be continuously passed to cool the vessel.

A flow diagram is given in FIGURE 11 of the drawings to show how thesevarious treatment steps may be correlated into a unitary process, itbeing understood that any individual treatment step diagrammaticallyillustrated in the flow diagram may be of the character illustratedabove for FIGURES 3 to 10 or may take other forms. As illustrated thesand or other bed material with or without pre-treatment is fedcontinuously into the reaction zone into which the reactants oxygen andsilicon tetrachloride are introduced. If the bed material is not fedcontinuously, it may be purged of accumulated silicon dioxide from timeto time and replaced.

- The product gases from the reaction zone entrain the silica and may becooled and then separated. The silica product thus separated is purifiedby blowing air or oxygen therethrough while heating, which gas may ormay not contain added ammonia depending on the conditions of operation.The silica may be sent to a grinding or dressing operation and then tostorage.

The flow diagram thus illustrates a variety of mutually cooperatingsteps in processes for producing oxides of silicon, by the oxidation ofsilicon tetrachloride.

The following examples are, given for the purpose of illustrating theinvention; all flow rates of gas are calculated on'the basis ofatmospheric conditions of temperature and pressure.

Example 1 The reactor consisted of a vertical shaft furnace 1,substantially as illustrated in FIGURE 3 and having an internal diameterof 15 inches and an overall height of 7 feet. It was lined with.chlorine-resistant brickwork 2 of thickness 9 inches, and insulated bybrickwork 3 of thickness 3 inches on the outside, thew hole beingcontained within a steel shell with openings 6 and 7 corresponding tothe vertical shaft.

The opening 7 at the base was sealed by an apertured plate 10substantially as illustrated in FIGURE 3, sup porting a block ofchlorine-resistant concrete of thickness 9 inches and, having seventeenpassages 112, 212 and 312 uniformly spaced as shown in plan in FIGURE 4,corresponding to seventeen apertures 13 in the plate 10. On the underside of the plate 10 ducting and manifolds were installed ashereinbefore described with reference to FIGURES 3 and 5. The silicontetrachloride vapour constrictions in the inlet means were of diameterinch, whereas the oxygen constrictions were of diameter inch. Y

The top *6 of the furnace 1 was sealed with an insulated plate 40 ofthickness 6 inches carrying a port 24, serving as a feed inlet for thesubstrate material comprising the bed to be fluidized, and also servingfor the insertion of a gas poker for preheating the bed; a second port126 in the wall of the furnace served for conducting the products ofreaction from the furnace.

The inclined conduit 77 was in this case sealed at its flange.

In the operation of this plant, silica sand of average diameter 250microns was fed into the reactor in such quantity that the depth of bedwhen fluidised was about 36 inches. The sand was fluidised by air fedvia the manifold system to all seventeen passages. By insertion of apre-ignited gas poker through the port 24, the bed was pre-heated to atemperature of 1250* C. At this stage, the gas poker was removed and theport 24 was sealed. Meanwhile, the air supply was substituted by anoxygen supply through the manifold ZS leading to the central passage andthe outer ring of passages at the rate of litres per minute and, as aprecaution, nitrogen was fed through the manifold 26 leading to theeight inner passages (through which the silicon tetrachloride isintended to flow) in order to free the whole of the inlet means fromoxygen and oxygen-containing gases. The nitrogen stream was thenarrested and replaced by silicon tetrachloride, to be led into thealready fluidised bed. Liquid silicon tetrachloride was measured at therate of 375 cc. per minute into a steam-jacketed vapourising tubewherein it was converted completely to (gaseous form and Was thereafterled into the fluidised bed reaction zone in the aforementioned manner.The molar ratio of silicon tetrachloride to oxygen was 1:2 and, althoughthis was maintained, there were minor adjustments in the feed rate ofthe reactants to maintain the temperature at 1000-l050 C., within theperiod of operation, i.e. 5 hours. The silicon tetrachloride reactedwith the oxygen within the bed to produce chlorine and silicon dioxide,the latter being removed from the bed through port 126 in an entrainedstream which was conveyed through cooling and separation units, wherebythe silicon dioxide was collected and the chlorine subsequently absorbedin sulphur chloride for regeneration.

' The silicon dioxide product had a particle size of about 0.002 micron.

Example 2 In this instance, the reactorwas similar in construction tothat used in Example 1, but with the following differences.

The internal diameter was 18 inches and overall height was 7 feet. Thediameter of the constrictions in the manifold system were for silicontetrachloride admission inch, and for the oxygen admisison inch.

' The inclined conduit 77 shown in'FIGURE 3 was lined withchlorine-resistant brickwork of thickness 3 inches, and was positionedat a height of about '40 inches from the bottom of the furnace.

Silica sand of average diameter 250 microns was fed by means of a beltlift at a controlled rate of the order of 26 lbs. per hour to the top ofthe solids-feeding device as shown in the top left portion of FIGURE 3.The sand thus fed accumulated above the perforated plate 73 and wasbrought into a fluidised state, and to an expanded height of about 2 /2feet on the side of the baffle 76 remote from the exit duct 70, by meansof compressed pipe 72 entering the bottom thereof. A portion of theexpanded bed overflowed via duct 70 to enter the furnace and the sandwas fed at a rate suflicient to control the reaction temperature. Theheight of the fluidised bed in the furnace 1 was established at about 40inches by means of overflow through the inclined conduit. The bed withinthe furnace 1 was continuously renewed, portions thereof overflowing asaforesaid and fresh bed material being admitted to the furnace from thesolids feed device via conduit 70. 1 p 7 Such bed material as overflowedfrom the furnace 1 passed down into vessel 81, therein to accumulateabove the perforated plate 82, and was fluidised by passing a currentofcompressed air into the vessel through pipe 85 located at its base.This treatment effectively removed from'the sand any residual traces ofchlorine or other undesired gases. When the sand which had accumulatedin vessel 81 was fully fluidised, portions thereof overflowed at aconstant rate through pipe 83.

With the sand feed suspended, the bed was preheated to about 1200 C. asin Example 1. Oxygen was supplied at a rate of 209 litres per minute,and silicon tetrachloride liquid was metered at the rate of 654 cc. per

gas, and then subjected to heat-treatment to remove therefrom adsorbedchlorine and/ or hydrochloric acid.

The fine silicon dioxideso obtained after the heattreatment consisted ofa finely-divided product having an average particle size of less than0.005 micron and a bulk density of 5 lbs. per cu. ft.

Example 3 In this instance the reactor was of the same construction anddimensions as in Example 2.

The silica sand constituting the bed had an average diameter of 250microns and was fed into the furnace to a fluidised depth of 40".- Theoxygen was supplied at a rate of 209 litres per minute measured at roomtemperature. The silicon tetrachloride liquid was metered at the rate of654 cc. per minute, also at room temperature, through a steam jacketedvapourising tube. The molar ratio of silicon tetrachloride to oxygen was1:15. In this instance the oxygen used had additionally a moisturecontent of 1.3% molar with respect to oxygen, this moisture-contentbeing obtained by bleeding off prior to admission 11 litres per minuteof the 209 litres per minute total oxygen stream, and bubbling these 11litrm per minute through water contained in two steel vessels eachcontaining three foot six inches depth of water, maintained at 70 C.

The temperature of the reactor was maintained at 1000-1050" C. during a7-hour period of operation by continuous slow replacement of the silicasand substrate in the reactor as described below.

Utilising the bed material. feeding system as shown in FIGURE 3' themodus operandi of such cooling system employed in the example was asfollows:

By means of an insulated filament wound in the form of a helix round theoutside of vessel 71, the latter was heated electrically by a circuitproviding 5 kw. of power. The cold sand was fed at the rate of 40 lbs.per hour to the topof. the solids-feeding device and was heated andfluidised within thevessel 71 by means already described, i.e. bycompressed air admitted at the rate of 130 litres per minute through thepipe 72 entering through the bottom and through the perforated plate 73.The temperature of the sand was controlled at about 400 C., andoveraoaaeeo flowed through the conduit 70 into the furnace 1. In

the bed was continuously renewed so as to avoid excessive build-up ofreaction products onto the substrate, the surplus substrate overflowingas previously described through the inclined conduit 77.

The silicon dioxide product discharged from the furnace 1 through theport 126 was collected in an agglomerated condition after cooling in acyclone.

It had an average size of about 0.004 micron. The bulk density of theagglomerated material was, 4% lbs. per cu. ft. After heat treatment,when two grams of this silica product was shaken with 20 cc. of waterthe suspension had a pH value of 4.1 as compared with a pH of 2.2 beforethe heat treatment. It had a surface area of 260 sq. meters per gram asmeasured by the B.E.T. method.

We claim:

1. Process for the manufacture of silicon dioxide of use in the rubberindustry as a fillerby oxidation in the absence of any substantialhydrolysis, of vapourised silicon tetrachloride with a gas comprisingfree oxygen, as

substantially entirely the'only reactants, comprising feed ing thereactantsinto a bed consisting of a hot mass of particulate solid inertmaterial having a mean particle size fromabout 40, to about 1,000 thevelocity of feed of at least one of said reactants being suflicient tomain-,

tain said bed in a fluidised state, so thatat a temperature in the rangefrom about 500 C. to about 1,300 C. said silicon tetrachloride and saidfree oxygen react within said bed to form silicon dioxide and chlorinewhich are continuously delivered from the bed, the silicon dioxideconsisting of aggregates of fine particles being in a fine flocculentstate entrained in the chlorine, substantially all of the'silicondioxide so produced being entrained in the product gases, and thereafterseparating the silicon dioxide from the chlorine.

2. Process of claim 1 in which the solid inert material is a substanceof the group consisting of silica, alumina, zircon and rutile, andmixtures thereof.

3. Process for the manufacture of silicon dioxide of use in the rubberindustry as a filler by oxidation, in the absence of any substantialhydrolysis, of vapourised silicon tetrachloride with a gas comprisingfree oxygen, as

substantially entirely the only reactants, comprising feedticulate.solid inert material having a mean particle sizeof from about 40 toabout 1,000,u, with a velocity, at least in respect of the air,sufficient to maintain said bed in a fluidised state, so that at atemperature in the range from about 500 C. to about l,300- C. saidsilicon tetrachloride and free oxygen of said air react within said bedto form silicon dioxide and chlorine which are continuously deliveredfrom the top portion of the bed, the silicon dioxide consisting ofaggregates of fine particles being. in a fine, flocculent stateentrained in the chlorine, substantially all of the silicon dioxide soproduced being entrained in the product gases, and thereafter separatingthe SlllCOIl dioxide from the chlorine.

References Cited in the file of this patent UNITED STATES PATENTS2,400,907 Behrman May 28, 1946 2,503,788 White Apr. 11, 1950 2,760,846Richmond et al. Aug. 28, 1956 2,798,792 Stellinget al. July 9, 19572,823,982 Saladin et al. Feb. 18, 1958 2,828,187 Evans et a1 Mar. 25,1958 2,841,476 Dalton July 1, 1958 2,863,738 Antwerp Dec. 9, 1958FOREIGN PATENTS 165,589 Australia Oct. 13, 1955

1. PROCESS FOR THE MANUFACTURE OF SILICON DIOXIDE OF USE IN THE RUBBERINDUSTRY AS A FILLER BY OXIDATION IN THE ABSENCE OF ANY SUBSTANTIALHYDROLYSIS, OF VAPOURISED SILICON TETRACHLORIDE WITH GAS COMPRISING FREEOXYGEN, AS SUBSTANTIALLY ENTIRELY THE ONLY REACTANTS, COMPRISING FEEDINGTHE REACTANTS INTO AS BED CONSISTING OF A HOT MASS OF PARTICULATE SOLIDINERT MATERIAL HAVING A MEAN PARTICLE SIZE FROM ABOUT 40U TO ABOUT1,000U THE VELOCITY OF FEED OF AT LEAST ONE OF SAID REACTANTS BEINGSUFFICIENT TO MAINTAIN SAID BED IN A FLUIDISED STATE, SO THAT AT ATEMPERATURE IN THE RANGE FROM ABOUT 500*C. TO ABOUT 1,300C. SAID SILICONTETRACHLORIDE AND SAID FREE OXYGEN REACT WITHIN SAID BED TO FORM SILICONDIOXIDE AND CHLORINE WHICH ARE CONTINUOUSLY DELIVERED FROM THE BED, THESILICON DIOXIDE CONSISTING OF AGGREGATES OF FINE PARTICLES BEING IN AFINE FLOCCULENT STATE ENTRAINED IN THE CHLORINE, SUBSTANTIALLY ALL OFTHE SILICON DIOXIDE SO PRODUCED BEING ENTRAINED IN THE PRODUCT GASES,AND THEREAFTER SEPARATING THE SILICON DIOXIDE FROM THE CHLORINE.